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Transcript of Energy Conversion System
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ENERGY CONVERSION TECHNOLOGY
Wind Energy Conversion System
1.0 Introduction
Differential heating of the earth's surface by the sun causes the movement of large air masses
on the surface of the earth, i.e., the wind. Wind energy conversion systems convert the kinetic
energy of the wind into electricity or other forms of energy. Wind power generation has
experienced a tremendous growth in the past decade, and has been recognized as an
environmentally friendly and economically competitive means of electric power generation.
2.0 Structure of Wind Energy Conversion Systems
The major components of a typical wind energy conversion system include a wind turbine,
generator, interconnection apparatus and control systems, as shown in Figure 1. Wind
turbines can be classified into the vertical axis type and the horizontal axis type. Most modern
wind turbines use a horizontal axis configuration with two or three blades, operating either
down-wind or up-wind. The major components in the nacelle of a typical wind turbine are
illustrated in Figure 4. A wind turbine can be designed for a constant speed or variable speed
operation. Variable speed wind turbines can produce 8% to 15% more energy output as
compared to their constant speed counterparts, however, they necessitate power electronic
converters to provide a fixed frequency and fixed voltage power to their loads. Most turbine
manufacturers have opted for reduction gears between the low speed turbine rotor and the
high speed three-phase generators. Direct drive configuration, where a generator is coupled to
the rotor of a wind turbine directly, offers high reliability, low maintenance, and possibly low
cost for certain turbines. Several manufacturers have opted for the direct drive configuration
in the recent turbine designs. At the present time and in the near future, generators for wind
turbines will be synchronous generators, permanent magnet synchronous generators, and
induction generators, including the squirrel cage type and wound rotor type. For small to
medium power wind turbines, permanent magnet generators and squirrel cage induction
generators are often used because of their reliability and cost advantages. Induction
generators, permanent magnet synchronous generators and wound field synchronous
generators are currently used in various high power wind turbines. Interconnection apparatus
are devices to achieve power control, soft start and interconnection functions. Very often,
power electronic converters are used as such devices. Most modern turbine inverters are
forced commutated PWM inverters to provide a fixed voltage and fixed frequency output
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with a high power quality. Both voltage source voltage controlled inverters and voltage
source current controlled inverters have been applied in wind turbines. For certain high power
wind turbines, effective power control can be achieved with double PWM (pulse width
modulation) converters which provide a bi-directional power flow between the turbine
generator and the utility grid.
Figure 1: Structure of a typical wind energy system.
Figure 2: Major components inside the nacelle of a turbine.
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Table 1: Costs of Various Wind Power Plants
3.0 SOLAR ENERGY CONVERSION TECHNOLOGY
3.1 Solar Technology Overview
Figure 3: Solar energy conversion paths and Technologies
A wide variety of solar technologies have the potential to become a large component of the
future energy portfolio. Passive technologies are used for indoor lighting and heating of
buildings and water for domestic use. Also, various active technologies are used to convert
solar energy into various energy carriers for further utilization:
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Photovoltaic directly converts photon energy into electricity. These devices use inorganic
or organic semiconductor materials that absorb photons with energy greater than their
bandgap to promote energy carriers into their conduction band. Electron-hole pairs, or
excitons for organic semiconductors, are subsequently separated and charges are collected at
the electrodes for electricity generation.
Solar thermal technologies convert the energy of direct light into thermal energy using
concentrator devices. These systems reach temperatures of several hundred degrees with high
associated exergy. Electricity can then be produced using various strategies including thermal
engines (e.g. Stirling engines) and alternators, direct electron extraction from thermionic
devices, Seebeck effect in thermoelectric generators, conversion of IR light radiated by hot
bodies through thermophotovoltaic devices, and conversion of the kinetic energy of ionized
gases through magnetohydrodynamic converters.
Photosynthetic, photo (electro)chemical, thermal, and thermochemical processes are used to
convert solar energy into chemical energy for energy storage in the form of chemical fuels,
particularly hydrogen. Among the most significant processes for hydrogen production are
direct solar water splitting in photo electrochemical cells or various thermochemical cycles
such as the two-step water-splitting cycle using the Zn/ZnO redox system
3.2 Photon-to-Electric Energy Conversion
A solar cell, or photovoltaic cell (PV), is a device that converts light into electric current using the
photoelectric effect.Solar cells produce direct current (DC) power which fluctuates with thesunlight's intensity. For practical use this usually requires conversion to certain desired
voltages or alternating current (AC), through the use of inverters.
Multiple solar cells are
connected inside modules. Modules are wired together to form arrays, then tied to an inverter,
which produces power at the desired voltage, and for AC, the desired frequency/phase. Many
residential systems are connected to the grid wherever available, especially in developed
countries with large markets.[16]
In these grid-connected PV systems, use of energy storage is
optional. In certain applications such as satellites, lighthouses, or in developing countries,
batteries or additional power generators are often added as back-ups. Such stand-alone power
systems permit operations at night and at other times of limited sunlight.Photovoltaic devices allow the direct production of electricity from light absorption. The
active material in a photovoltaic system is a semiconductor capable of absorbing photons
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with energies equal to or greater than its bandgap. Upon photon absorption, an electron of the
valence band is promoted to the conduction band and is free to move through the bulk of the
semiconductor. In order for this free charge to be captured for current generation, decay to
the lower energy state, i.e. recombination with the hole in the valence band, has to be
prevented through charge separation.
In photovoltaic devices made of inorganic semiconductors, charge separation is driven by the
built-in electric field at the p-n junction. As a consequence, their efficiency is determined by
the ability of photon generated minority carriers to reach the p-n junction before recombining
with the majority carriers in the bulk of the material. Thus, bulk properties such as
crystallinity and chemical purity often control the device efficiency.
Figure 4: Schematic of a p-n junction and of an organic bilayer structure
In both inorganic and organic photovoltaic technologies, many strategies are under
investigation for achieving efficient light absorption, charge separation, transport, and
collection. The strategies are based on technologies that involve the use inorganic
semiconductor materials such as silicon (c-Si, pc-Si, or -Si), III-V compounds (e.g. GaAs,
InP), chalcogenides (e.g. CdTe, CIGS), and various organic-based thin films:
Additionally, advanced thin-film technologies, called 3rd
generation photovoltaics, are
considered as a promising route to increasing the efficiency and/or lowering the cost of
photovoltaics.
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3.3 Photon-to-Thermal-to-Electric Energy Conversion
This section involves solar thermal technologies that produce electricity through
concentration of solar energy for the production of heat and subsequent conversion into
electric current. There are a number of options available at different stages of development.
The most developed technologies are the parabolic dish, the parabolic trough, and the power
tower.
The parabolic dish is already commercially available. This system is modular and can be used
in single dish applications (with output power of the order of 25kWe) or grouped in dish
farms to create large multi-megawatt plants.
Parabolic troughs are a proven technology and will most likely be used for deployment of
solar energy in the near-term. Various large plants are currently in operation (California -
354MW) or in the planning process in the USA and in Europe.
Power towers, with low cost and efficient thermal storage, promise to offer dispatchable, high
capacity factor power plants in the future. Together with dish/engine systems, they offer the
opportunity to achieve higher solar-to-electric efficiencies and lower cost than parabolic
trough plants (see Table 2), but uncertainty remains as to whether these technologies can
achieve the necessary capital cost reductions.
Table 2: Characteristics of major solar thermal electric power systems
Parabolic troughs
Parabolic trough systems use single-axis tracking parabolic mirrors to focus sunlight on
thermally efficient receiver tubes that contain a heat transfer fluid (HTF). The receiver tubes
are usually metallic and embedded into an evacuated glass tube that reduces heat losses. A
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special high-temperature coat
(e.g. thermo oil) is heated to
produce superheated steam w
produce electricity. It is als
collectors. This makes the
relatively expensive thermo
direct steam generation (DS
solve the thermo-mechanical
presence of a two-phase fluid
The efficiency of a solar ther
efficiency and steam-cycle
incidence of the sunlight and
75%. Field losses are usually
reach annual efficiencies of a
significant influence. Central
higher temperatures and there
Figure 5: A diagram of a para
parabolic collector focuses su
7
ng additionally reduces radiation heat losses
~ 400oC and pumped through a series of
ich powers a conventional turbine generator
o possible to produce superheated steam
hermo oil unnecessary, and also reduces
oil and the heat exchangers are no longer
) is still in the prototype stage and more res
issues related to working pressures abov
in the receivers.
mal power plant is the product of the collec
fficiency. The collector efficiency depend
the temperature in the absorber tube, and ca
below 10%. Altogether, solar thermal troug
out 15%; the steam-cycle efficiency of abou
receiver systems such as solar thermal tow
fore achieve higher efficiencies.
olic trough solar farm (top), and an end vie
nlight onto its focal point.
. The working fluid
heat exchangers to
(Rankine cycle) to
irectly using solar
costs because the
needed. However,
arch is required to
100 bar and the
tor efficiency, field
s on the angle of
reach values up to
h power plants can
t 35% has the most
r plants can reach
of how a
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Power towers
In a power tower plant, hundreds of two-axis tracking heliostats are installed around a tower
where they focus sunlight with concentrations ranging from 100 to 10,000 suns. The absorber
is located on the top of the tower and can reach temperatures from 200 oC to 3000oC]. Hot air
or molten salt are usually used to transport the heat from the absorber to a steam generator
where superheated steam is produced to drive a turbine and an electrical generator. Power
towers are suited for large-output applications, in the 30 to 400MWe range, and need to be
large to be economical. Thermal storage can be easily integrated with this type of solar
systems, allowing the enhancement of the annual capacity factor from 25% to 65% and the
stabilization of the power output through fluctuations in solar intensity until the stored energy
is depleted.
Since the early 1980s, power towers were built in Russia, Italy, Spain, Japan, France, and the
USA, with power outputs ranging from 0.5MWe to 10MWe (Solar Two, Southern California)
and using various combinations of heat transfer fluids (steam, air, liquid sodium, molten
nitrate, molten nitrate salt) and storage media (water/steam, nitrate salt/water, sodium,
oil/rock, ceramic)
The efficiency of a solar-powered steam turbine electric generator used in the power towerconcept is a critical function of the temperature TR of the receiver, which is influenced not
only by the incident energy but also of several factors including the heliostat optical
performance, the mirror cleanliness, the accuracy of the tracking system, and wind effects.
Table 3 Properties of the principal HTFs for parabolic troughs and power towers
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The development of new heat transfer fluids (HTFs) is crucial for increasing the operating
temperature of a solar thermal plant, and hence the efficiency of the steam cycle. Stability at
high temperature, low flammability, low vapor pressure at high temperature, low corrosivity
in standard materials, low freezing point, high boiling point, and low cost are the main
required characteristics. Table 2 lists the operating temperature and the main characteristics
of some HTFs considered for parabolic troughs and power towers.
Dish-engine systems
Dish-engine systems can be used to generate electricity in the kilowatts range. A parabolic
concave mirror concentrates sunlight; the two-axis tracked mirror must follow the sun with a
high degree of accuracy in order to achieve high efficiencies. At the focus is a receiver
whichis heated up over 700C. The absorbed heat drives a thermal engine which converts the
heat into motive energy and drives a generator to produce electricity. If sufficient sunlight is
not available, combustion heat from either fossil fuels or biofuels can also drive the engine
and generate electricity. The solar-to-electric conversion efficiency of dishengine systems
can be as high as 30%, with large potential for low-cost deployment. For the moment, the
electricity generation costs of these systems are much higher than those for trough or tower
power plants, and only series production can achieve further significant cost reductions for
dishengine systems. A number of prototype dish-engine systems are currently operating in
Nevada, Arizona, Colorado, and Spain. High levels of performance have been established;
durability remains to be proven, although some systems have operated for more than 10,000
hours.
3.4 Photon-to-Chemical Energy Conversion
Photoconversion processes are used for producing a large variety of chemicals with clear
energetic and environmental advantages compared to conventional technical processes. This
section focuses on the synthesis of chemical fuels e.g. ammonia, methane, or hydrogen
since this application has the largest potential in terms of energy production. Moreover, it
could partially solve one of the principle shortcomings of conventional solar technologies,
which is the lack of capacity for energy storage. Among the large variety of identified
processes and technologies, we consider here three main categories of solar-to-chemical
conversion processes: photo(electro)chemical processes, thermochemical processes, and
photosynthetic processes in natural systems. Photochemical and photoelectrochemicalsystems use light-sensitive materials (in aqueous suspension or in the form of bulk electrodes,
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respectively) for absorbing photon energy and producing electrons with sufficient energy for
splitting water. In thermochemical technologies, concentrated solar flux is used to produce
the high-temperatures necessary to drive endothermic reactions such as syngas production
from natural gas, water thermal decomposition, and water splitting through high-temperature
chemical cycles.
4.0 Hydropower Technologies
Hydropower transforms the potential energy of a mass of water flowing in a river or stream
with a certain vertical fall (termed the head10). The potential annual power generation of a
hydropower project is proportional to the head and flow of water. Hydropower plants use a
relatively simple concept to convert the energy potential of the flowing water to turn a
turbine, which, in turn, provides the mechanical energy required to drive a generator and
produce electricity (Figure 6).
Figure 6: Typical low head hydropower plant with storage
The main components of a conventional hydropower plant are:
Dam: Most hydropower plants rely on a dam that holds back water, creating a large water
reservoir that can be used as storage. There may also be a de-silter to cope with sediment
build-up behind the dam.
Intake, penstock and surge chamber: Gates on the dam open and gravity conducts the
water through the penstock (a cavity or pipeline) to the turbine. There is sometimes a head
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race before the penstock. A surge chamber or tank is used to reduce surges in water pressure
that could potentially damage or lead to increased stresses on the turbine. The penstock
conveys water under pressure to the turbine and can be made of, or lined with, steel, iron,
plastics, concrete or wood. The penstock is sometimes created by tunnelling through rock,
where it may be lined or unlined.
Turbine: The water strikes the turbine blades and turns the turbine, which is attached to a
generator by a shaft. There is a range of configurations possible with the generator above or
next to the turbine.
Turbines are devices that convert the energy from falling water into rotating shaft power.
There are two main turbine categories: reactionary and impulse. Impulse turbines extract
the energy from the momentum of the flowing water, as opposed to the weight of the water.
Reaction turbines extract energy from the pressure of the water head. The most suitable and
efficient turbine for a hydropower project will depend on the site and hydropower scheme
design, with the key considerations being the head and flow rate. The Francis turbine is a
reactionary turbine and is the most widely used hydropower turbine in existence. Francis
turbines are highly efficient and can be used for a wide range of head and flow rates. The
Kaplan reactionary turbine was derived from the Francis turbine but allows efficienthydropower production at heads between 10 and 70 metres, much lower than for a Francis
turbine. Impulse turbines such as Pelton, Turgo and cross-flow (sometimes referred to as
Banki-Michell or Ossberger) are also available. The Pelton turbine is the most commonly
used turbine with high heads. Banki-Michell or Ossberger turbines have lower efficiencies
but are less dependent on discharge and have lower maintenance requirements.
Generators: As the turbine blades turn, the rotor inside the generator also turns and electric
current is produced as magnets rotate inside the fixed-coil generator to produce alternating
current (AC). There are two types of generators that can be used in small hydropower plants:
asynchronous (induction) and synchronous machines. Asynchronous generators are generally
used for micro-hydro projects.
Transformer: The transformer inside the powerhouse takes the AC voltage and converts it
into higher-voltage current for more efficient (lower losses) long-distance transport.
Transmission lines: Send the electricity generated to a grid-connection point, or to a largeindustrial consumer directly, where the electricity is converted back to a lower-voltage
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current and fed into the distribution network. In remote areas, new transmission lines can
represent a considerable planning hurdle and expense.
Outflow: Finally, the used water is carried out through pipelines, called tailraces, and re-
enters the river downstream. The outflow system may also include spillways which allow
the water to bypass the generation system and be spilled in times of flood or very high
inflows and reservoir levels.
5.0 GEOTHERMAL ENERGY
Geothermal is, simply, heat from the Earth. It is a clean, renewable resource that provides
energy in the United States and around the world. It is considered renewable because the
heat emanating from the interior of the Earthgeothermal energyis essentially limitlessand is constantly being regenerated. The Earths interior is expected to remain extremely hot
for billions of year to come, generating heat equivalent to 42 million megawatts of power.
If geothermal power plants are managed properly, they can produce electricity for decades or
more.
GEOTHERMAL FLUID
Geothermal fluida hot, sometimes salty, mineral-rich liquid and/or vaporis the carrier
medium that brings geothermal energy up through wells from the subsurface to the surface.
This hot water and/or steam is withdrawn from a deep underground reservoir and isolated
during production, flowing up wells and converting into electricity at a geothermal power
plant. Once used, the water and condensed steam is injected back into the geothermal
reservoir to be reheated. It is separated from groundwater by thickly encased pipes, making
the facility virtually free of water pollution.
A resource that uses an existing accumulation of hot water or steam is known as a
hydrothermal resource. While several other types of geothermal resources exist, all
producing geothermal plants in the United States use hydrothermal resources. Characteristics
of the geothermal fluid, including temperature, chemistry, and non-condensable gas content
(NCG), which can influence power plant design.
POWER PLANT BASICS
Like all conventional thermal power plants, a geothermal plant uses a heat source to expand a
liquid to vapor/steam. This high pressure vapor/steam is used to mechanically turn a turbine-
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generator. At a geothermal plant, fuel is geothermal water heated naturally in the earth, so no
burning of fuel is required. At many power plants, a steam turbine is used to convert the
thermal energy extracted from pressurized steam into useful mechanical energy. Mechanical
energy is then converted into electricity by the generator.
Geothermal plants rely upon one or a combination of three types of conversion technology
binary, steam, and flash to utilize the thermal energy from the hot subsurface fluids and
produce electricity.
CONVERSION TECHNOLOGIES
A conversion technology represents the entire process of turning hydrothermal resources into
electricity. Of the four available to developers, one of the fastest growing is the binary cycle,
which includes a Rankine cycle engine.
I. Steam
Dry steam plants have been operating for over one hundred yearslonger than any other
geothermal conversion technology, though these reservoirs are rare. In a dry steam plant like
those at The Geysers in California, steam produced directly from the geothermal reservoir
runs the turbines that power the generator. Dry steam systems are relatively simple, requiring
only steam and condensate injection piping and minimal steam cleaning devices. A dry
steam system requires a rock catcher to remove large solids, a centrifugal separator to remove
condensate and small solid particulates, condensate drains along the pipeline, and a final
scrubber to remove small particulates and dissolved solids. Today, steam plants make up a
little less than 40 percent of U.S. geothermal electricity production, all located at The Geysers
in California.
The basic cycle for steam plants remains similar to the structure that first operated in 1904 in
Larderello, Italy, pictured in the figure above. Even so, incremental technology
improvements continue to advance these systems. Figure 7 shows a dry steam plant.
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Figure 7: Dry Steam Power Plant Diagram
II. Flash
The most common type of power plant to date is a flash power plant, where a mixture of
liquid water and steam is produced from the wells. About 45 percent of geothermal
electricity production in the U.S. comes from flash technology. At a flash facility, hot liquid
water from deep in the earth is under pressure and thus kept from boiling. As this hot water
moves from deeper in the earth to shallower levels, it quickly loses pressure, boils and
flashes to steam. The steam is separated from the liquid in a surface vessel (steam
separator) and is used to turn the turbine, and the turbine powers a generator. Flash power
plants typically require resource temperatures in the range of 350 to 500oF (177
oC to 260
oC).
A number of technology options can be used with a flash system. Double flashing, the most
popular of these, is more expensive than a single flash, and could concentrate chemical
components if they exist in the geothermal water. Even considering potential drawbacks,
most geothermal developers agree that double flash is more effective than single flash
because a larger portion of the resource is used.
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Figure 8: Well Flowing Steam through a Silencer at Coso, a Double Flash Plant in California
Steam processing is an integral part of the gathering system for flash and steam plants. In
both cases, separators are used to isolate and purify geothermal steam before it flows to the
turbine. A flash system requires three or more stages of separation, including a primary flash
separator that isolates steam from geothermal liquid, drip pots along the steam line, and a
final polishing separator/scrubber. A steam wash process is often employed to further
enhance steam purity. All geothermal power plants require piping systems to transport water
or steam to complete the cycle of power generation and injection.
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Figure 9: Single Flash Steam Power Plant Schematic
Figure 10: Double Flash Steam Power Plant Schematic
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III. Binary
Technology developments during the 1980s have advanced lower temperature geothermal
electricity production. These plants, known as binary geothermal plants, today make use of
resource temperatures as low as 165oF, or 74oC (assuming certain parameters are in place)
and as high as 350oF (177
oC). Approximately 15 percent of all geothermal power plants
utilize binary conversion technology.
In the binary process, the geothermal fluid, which can be either hot water, steam, or a
mixture of the two, heats another liquid such as isopentane or isobutane (known as the
working fluid), that boils at a lower temperature than water. The two liquids are kept
completely separate through the use of a heat exchanger used to transfer heat energy from
the geothermal water to the working fluid. When heated, the working fluid vaporizes into
gas and (like steam) the force of the expanding gas turns the turbines that power the
generators.
Figure 10: Binary Power Plant Schematic
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6.0 Biomass Energy Conversion
6.1. Biomass conversion processes
The development of conversion technologies for the utilization of biomass resources for
energy is growing at a fast pace. Most developing countries find it hard to catch up because
the level of technology is beyond their manpower as well as their manufacturing and
technological capability. Added to this is the unavailability of local materials and parts for the
fabrication of these conversion units. Figure 1 shows the different methods for converting
biomass into convenient fuel. Biomass conversion into heat energy is still the most efficient
process but not all of energy requirement is in the form of heat. Biomass resources need to be
converted into chemical, electrical or mechanical energy in order to have widespread use.
These take the form of solid fuel like charcoal, liquid fuel like ethanol or gaseous fuel like
methane. These fuels can be used in a wide range of energy conversion devices to satisfy the
diverse energy needs. In general, conversion technologies for biomass utilization may either
be based on bio-chemical or thermo-chemical conversion processes. Each process will be
described separately.
Figure 11: Methods of using Biomass Energy
6.1 Bio-chemical conversion processes
The two most important biochemical conversion processes are the anaerobic digestion and
fermentation processes.
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6.1.1 Anaerobic digestion
Anaerobic digestion is the treatment of biomass with naturally occurring microorganisms in
the absence of air (oxygen) to produce a combustible gaseous fuel comprising primarily of
methane (CH4) and carbon dioxide (CO2) and traces of other gases such as nitrogen (N2) and
hydrogen sulphide (H2S). The gaseous mixtures is commonly termed biogas. Virtually all
nitrogen (N), phosphorus (P) and potassium (K) remain in the digested biomass. The entire
process takes place in three basic steps as shown in Figure 2. The first step is the conversion
of complex organic solids into soluble compounds by enzymatic hydrolysis. The soluble
organic material formed is then converted into mainly short-chain acids and alcohols during
the acidogenesis step. In the methanogenesis step, the products of the second step are
converted into gases by different species of strictly anaerobic bacteria. The percentage of
methane in the final mixture has been reported to vary between 50 to 80%. Atypical mixture
consists of 65% methane and 35% CO2 with traces of other gases. The methane producing
bacteria (called methanogenic bacteria) generally require a pH range for growth of 6.4 to 7.2.
The acid producing bacteria can withstand low pH. In doing their work, the acid producing
bacteria lower the pH and accumulate acids and salts of organic acids. If the methane-
forming organisms do not rapidly convert these products, the conditions become adverse to
methane formers. This is why the first type of reactors developed for conversion of biomasswastes into methane have long retention times seeking equilibrium between acid and methane
formers. Municipal wastes and livestock manures are the most suitable materials for
anaerobic digestion. In the US, numerous landfill facilities now recover methane and use it
for power generation. Aquatic biomass such as water hyacinth or micro-algae can be digested
and may become valuable sources of energy in the future. Anaerobic digestion of organic
wastes may constitute an effective device for pollution control with simultaneous energy
generation and nutrient conservation. A major advantage of anaerobic digestion is that it
utilizes biomass with high water contents of as high as 99%. Another advantage is the
availability of conversion systems in smaller units. Also the residue has fertilizer value and
can be used in crop production. The primary disadvantage of anaerobic digestion of diluted
wastes is the large quantity of sludge that must be disposed of after the digestion process
including the wastewater and the cost of biogas storage. In cold climates, a significant
fraction of the gas produced may be used to maintain the reactor operating temperature.
Otherwise, microorganisms that thrive on lower or moderate temperatures should be used.
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6.2 Thermo-chemical conversion processes
Biomass wastes can be easily converted into other forms of energy at high temperatures,
They break down to form smaller and less complex molecules both liquid and gaseous
including some solid products. Combustion represents a complete oxidation to carbon dioxide
(CO2) and water (H2O). By controlling the process using a combination of temperature,
pressures and various catalysts, and through limiting the oxygen supply, partial breakdown
can be achieved to yield a variety of useful fuels. The main thermo-chemical conversion
approaches are as follows: pyrolysis/charcoal production, gasification and combustion. The
advantages of thermo-chemical conversion processes include the following:
a. Rapid completion of reactions
b. Large volume reduction of biomass
c. Range of liquid, solid and gaseous products are produced
d. Some processes do not require additional heat to complete the process
6.2.1 Pyrolysis
Pyrolysis or destructive distillation is an irreversible chemical change caused by the action of
heat in the absence of oxygen. Pyrolysis of biomass leads to gases, liquids and solid residues.
The important components of pyrolysis gas in most cases are hydrogen, carbon monoxide,
carbon dioxide, methane and lesser quantities of other hydrocarbons (C2H4, C2H6, etc.). The
liquid consists of methanol, acetic acid, acetone, water and tar. The solid residue consists of
carbon and ash. Thus pyrolysis can be used to convert biomass into valuable chemicals and
industrial feedstock.
In a typical pyrolysis process the feed material goes through the following operations: (a)
primary shredding (b) drying the shredded material (c) removal of organics (d) further
shredding to fine size (e) pyrolysis (f) cooling of the products to condense the liquids and (g)
storage of the products.
Different types of pyrolytic reactors include vertical shaft reactors, horizontal beds. Among
these, the simplest and generally cheapest is the vertical shaft type. Fluidized bed reactors
are relatively a recent development. Figure 11 shows a rotary kiln pyrolysis reactor. The unit
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is cylindrical, slightly inclined and rotates slowly which causes the biomass to move through
the kiln to the discharge end.
Numerous technologies have now been developed for the production of bio-oil and char
using the pyrolysis process, Many of the reactors developed are improvements on the
traditional reactors used in rural areas of developing countries that include simple pit kilns
or drum type reactors. The energy efficiency of charcoal production using these methods is
only the order of 17-29% while theoretically, efficiencies as high as 40% could be achieved.
Fig. 12. Schematic of the rotary kiln pyrolysis reactor
6.2.2 Gasification
Gasification is the thermo-chemical process of converting biomass waste into a low medium
energy gas utilizing sub-stoichiometric amounts of oxidant (Coovattanachai,1991).
The simplest form of gasification is air gasification in which biomass is subjected to
partial combustion with a limited supply of air. Air gasifiers are simple, cheap and
reliable. Their chief drawback is that the gas produced is diluted with nitrogen and hence
has low calorific value. The gas produced is uneconomical to distribute; it must be used
on-site for process heat. In oxygen gasification, pure oxygen is used so that the gas
produced is of high energy content. The chief disadvantage of oxygen gasification is that
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it requires an oxygen plant and thus increases the total cost of gasification. The schematic
diagram of the processes occurring in a gasifier is shown in Figure 12 including the
temperature profile at each important step in the process.
Fig. 12. Schematic diagram of processes occurring in a gasifier and the temperature profile.
6.2.3 Biomass combustion
One of the most common methods of biomass conversion is by direct combustion or
burning. The simplest units include numerous cookstoves already developed in rural areas
of developing countries. Much improved and continuous flow designs include the Spreader-
Stoker system used in many refuse derived fuels (RDF) facility for converting solid wastes,
and the fluidized bed combustion units (similar to that shown in Figure 12). The number
component parts of this system is listed below:1. Refuse charging hopper
2. Refuse charging throat
3. Charging ram
4. Grates
5. Roller bearings
6. Hydraulic power cylinders and control valves
7. Vertical drop-off
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8. Overfire air jets
9. Combustion air
10. Automatic sifting removal system
In a spreader-stoker system, the fuel is introduced into the firebox above a grate. Smaller
particles will tend to burn in suspension and larger pieces will fall onto the grate. Most
units, if properly designed, can handle biomass with moisture content as high as 50-55%.
Moisture contained in the fuel is driven off partially when the fuel is in suspension and
partially on the grate. The feed system should provide an even thin layer of fuel on the
grate.
In a fluidized bed combustor (FBC), the fuel particle burns in a fluidized bed of inert particles
utilizing oxygen from the air. Advantages of fluidized bed combustion include: (1) high heat
transfer rate, (2) increased combustion intensity compared to conventional combustors and,
(3)absence of fouling and deposits on heat transfer surfaces.
The schematic diagram of a fluidized bed combustor is similar to that of a fluidized bed
gasifier. The only difference is the use of excess air for combustion processes and starved air
for gasification processes. So far FBC has been used mostly for coals. A number of wastes,
e.g. wastes from coal mining and municipal wastes, are also sometimes incinerated in
fluidized beds. It has been suggested that certain quick-maturing varieties of wood could be
combusted in fluidized beds for generation of steam. There is indeed a global search for
suitable varieties of wood for this purpose and FBC is likely to play an important role in
supplying energy requirements in certain countries in the future.
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Fig. 13. Schematic diagram of a reciprocating grate combustor (Courtesy of Detroit
Reciprogate Stocker).
Granular biomass fuels, e.g. paddy husk and chips of wood up to 2cm x 2cm x 2cm in size
have been successfully combusted in fluidized beds of sand particles. Conventional
combustion of paddy husk is slow and inefficient. Nearly complete combustion and high
combustion intensities of paddy husk can be achieved in a fluidized bed combustor. The same
combustor can also be used for burning wood. Combustion intensities up to about 500
Kg/hr-m2 have been achieved in fluidized bed combustors using biomass fuels. A number of
thermo-chemical conversion processes exist for converting biomass into liquid fuels. These
can be crudely divided into direct liquefaction and indirect liquefaction (in which the biomass
is gasified as a preliminary step) processes. While all these techniques are relatively
sophisticated and will generally be suitable for large scale conversion facilities, they do
represent an important energy option for the future because the heavy premium
that liquid fuels carry. The steam produced from heat of combustion of biomass may power a
steam turbine to produce electricity. However, because of the high ash contents of most
biomass resources, direct combustion of these biomass resources is not practical and efficient
due to slagging and fouling problems. Because of these problems, some biomass with high
ash are often mixed with low ash biomass such as coal, also termed co-firing.
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6.2.4 Biomass co-firing
Co-firing refers to mixing biomass and fossil fuels in conventional power plants. Significant
reductions in sulphur dioxide (SO2 an air pollutant released when coal is burned) emissions
are achieved using co-firing systems in power plants that use coal as input fuel. Small-scale
studies at Texas A&M University show that co-firing of manure with coal may also reduce
nitrogen oxides (NOx- contribute to air pollution) emissions from coal (Carlin, 2009).
Manure contains ammonia (NH3). Upon co-firing manure and coal, NH3 is released from
manure and combines with NOx to produce harmless N and water. Biomass co-firing has the
potential to cut emissions from coal powered plants without significantly increasing the cost
of infrastructure investments (Neville, 2011). Research shows that when implemented at
relatively low biomass-to-coal ratios, energy consumption, solid waste generation and
emissions are all reduced. However, mixing biomass and coal (especially manure) does create
some challenges that must be address.
There are three types of co-firing systems adopted around the world as follows:
a. Direct co-firing
b. Indirect co-firing , and
c. Separate biomass co-firing.
Direct co-firing is the simplest of the three and the most common option especially if the
biomass have very similar characteristics with coal. In this process, more than one type of
fuel is injected into the furnace at the same time. Indirect co-firing involves converting the
biomass into gaseous form before firing. The last type has a separate boiler for the co-fired
fuel. It was reported that the carbon life cycle and energy balance when co-firing 15%
biomass with coal is carbon neutral or better (Eisenstat, et al., 2009). In this research, carbon
emissions are reduced by 18%.